As he'd hoped, the apothecary on Knockturn not only stocked the ingredients needed for polyjuice potion, but they also sold the finished potion in single-serve vials to gallon jugs, ostensibly for all the guests at an extreme costume party!
Roger purchased the half pint flask, boomslang skin, powdered bicorn horn, and a list of ingredients for his own stock as well as for potions he thought Miss Granger might be most likely to brew.
His brother looked too much like him, so Roger had snuck hairs from Milo Cavendish's comb in the locker room the day he'd been asked to leave practice. After a couple firewhiskey's at the Warlock's Wand, he stepped into the Gent's and slipped a hair into the flask.
The potion tasted especially vile, with a metallic aftertaste. He really would have to brew his own, though he sincerely hoped this Witch Weekly furor would blow over long before could get the potion made. Surveying the result in the gent's mirror, Roger felt confident enough to leave his hood down when he left the Wand.
Since he was no longer looked like Roger Davies and he didn't care a flip about Milo's reputation, he took his time strolling further down Knockturn into the seediest part of wizarding London.
"Ssssst!" The sound made Roger stop short and look around. He knew he shouldn't but Ravenclaw curiosity urged him to step into the tiny dark alleyway.
"There's 3 galleons worth," the scratchy voice said as a small bag was thrust his way.
Knowing he was being very very bad, Roger accepted the package with a grunt and dug out 3 galleons. Whatever it was had better be worth it! He had money, but 3 galleons was still a lot.
He saw one suspiciously raised eyebrow before the figure turned and disappeared further up the narrow alley. Knowing the real Milo Cavendish was likely to appear any second, Roger apparated to Glasgow and strolled about till he felt the polyjuice wear off. Then he apparated to his home in Portree and set to work.